Immunogenic compositions that include SIRS virus

ABSTRACT

The invention includes kits for producing and using immunogenic compositions that include swine infertility and respiratory virus. The invention also includes vaccines and sera for treatment of Mystery Swine Disease (MSD), a method for producing the vaccine, methods for diagnosis of MSD, a viral agent that will mimic “mystery swine disease” and antibodies to the viral agent useful in diagnosis and treatment of MSD. The serum contains mammalian antibodies which are effective in treating MSD.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/281,884, filed Oct. 28, 2002 and issued as U.S. Pat. No. 6,855,315 onFeb. 15, 2005; which is a continuation of U.S. application Ser. No.09/740,156, filed Dec. 18, 2000 and issued as U.S. Pat. No. 6,498,008 onDec. 24, 2002; which is a continuation of U.S. application Ser. No.09/494,869, filed Jan. 31, 2000 and issued as U.S. Pat. No. 6,241,990 onJun. 5, 2001; which is a continuation of U.S. application Ser. No.08/927,786, filed Sep. 11, 1997 and issued as U.S. Pat. No. 6,110,468 onAug. 29, 2000; which is a continuation of U.S. application Ser. No.08/316,529, filed Sep. 30, 1994 and issued as U.S. Pat. No. 5,846,805 onDec. 8, 1998; which is a division of U.S. application Ser. No.08/301,738, filed Sep. 6, 1994, now abandoned; which is a file wrappercontinuation of U.S. application Ser. No. 07/860,444, filed Mar. 30,1992, now abandoned; which is a continuation in part of Ser. No.07/749,839, filed Aug. 26, 1991, now abandoned, and a continuation inpart of Ser. No. 07/760,713, filed Sep. 16, 1991, now abandoned. All theaforementioned applications are incorporated herein by reference intheir entireties.

BACKGROUND

Since 1987, the swine-producing industry has been subjected to adevastating epidemic of an unknown disease, often referred to as“Mystery Swine Disease” [MSD, more recently referred to as “SwineInfertility and Respiratory Syndrome (SIRS)], because researchers havebeen unable to identify the causative agent. MSD has affected hundredsof thousands of swine throughout North America and Europe. Once one pigis infected with MSD, that one pig can spread the MSD to an entire herdwithin three to seven days. From 1987 to 1991, the swine industry haslost millions of dollars in revenue as a result of MSD. A recent studyestimates that MSD causes a financial loss between $250 and $500 perinventoried sow.

MSD causes multiple symptoms in swine. The first symptom of MSD in abreeding herd of swine is usually anorexia and mild pyrexia. Inaddition, the herd animals may exhibit bluish discolorations in theirskin, especially in their ears, teats, snout, and the frontal portionsof their necks and shoulders. The affected skin may become irreparablydamaged. However, the most devastating symptom of MSD is thereproductive failure that occurs in a breeding herd of swine. MSD causessows to bear stillborn piglets; undersized, weak piglets withrespiratory distress; or piglets which die before they are weaned. Otherreproductive symptoms caused by MSD include early farrowing of piglets,a decrease in conception rates, failure in some sows to cycle, and areduction in the total number of piglets found in a litter. It has beenestimated that the number of pigs lost from reproductive failure isabout 10 to 15 percent of the annual production of pigs.

Research has been directed toward isolating the causative agent of MSD.A number of potential bacterial pathogens have been isolated. However,the types of potential bacterial pathogens have varied betweenswine-producing farms. Viral investigation has included fluorescentantibody examination, electron microscopic investigation, and serology.These methods have failed to locate the causative agent of MSD. As aresult, no one has yet developed a vaccine which can be used to treatMSD in the swine population.

Therefore, it is an objective of the invention to provide a vaccine andsera which, when administered to a breeding swine herd, will reduce thepresence of MSD in their population. Another object is to provide amethod of treating a population of swine with the vaccine to eradicateMSD from the swine population. Yet another object is to provide a methodfor diagnosis of MSD.

SUMMARY

These and other objects are achieved by the present invention which isdirected to a vaccine and sera for prevention and treatment of mysteryswine disease and to a method for its diagnosis in swine.

The vaccine is derived from an infectious agent that will infect swinewith mystery swine disease (MSD). The infectious agent is obtained froman inoculum of processed tissue of swine infected with the disease,preferably lung tissue. Preferably, the infectious agent is the productof an in vitro mammalian cell culture such as a simian cell lineinfected with the inoculum of the infected swine tissue. Preferably, theinoculum contains biological particles no greater than about 1.0 micronin size, more preferably 0.5 micron, most preferably no greater than 0.2micron. It is also preferable that the inoculum has been neutralizedwith antibodies to common swine diseases.

According to the present invention, a tissue homogenate obtained frompiglets in SIRS-affected herds consistently reproduced the respiratoryand reproductive forms of SIRS when intranasally inoculated ingnotobiotic piglets and pregnant sows. Gnotobiotic piglets so inoculatedwith-either unfiltered or filtered (0.45, 0.22, or 0.1 μm) inoculumbecame anorectic and developed microscopic lung lesions similar tolesions seen in SIRS-affected herds. The same inoculum also causedreproductive effects identical to those seen in SIRS-affected herds. Aviral agent has been recovered from the tissue homogenate. The viralagent causes a disease that mimics SIRS in piglets and pregnant sows.The viral agent has not yet been classified. However, the viral agent isa fastidious, non-hemagglutinating enveloped RNA virus. A viral agentcausing SIRS has been deposited on Jul. 18, 1991 with the American TypeCulture Collection, 12301 Parklawn Drive, Rockville, Md. 20852 under theaccession number ATCC VR-2332.

The serum for treatment of infected swine carries mammalian antibodiesto the MSD. It is obtained from the blood plasma of a mammal (non-swineand swine) pre-treated with the above-described infectious agent.

Alternatively, the serum is formulated from monoclonal antibodies to MSDproduced by hybridoma methods.

The method for diagnosis of MSD is based upon the use ofimmununospecific antibodies for MSD. The method calls for combination ofa filtered homogenate of a lung biopsy sample or a biopsy sample orsimilar samples (homogenate or biopsy) from other tissue and theimmunospecific antibodies followed by application of a known detectiontechnique for the conjugate formed by this combination. Immobilizationor precipitation of the conjugate and application of such detectiontechniques as ELISA; RIA; Southern, Northern, Western Blots and the likewill diagnose MSD.

According to the present invention, therapeutic and diagnostic methodsemploying antibodies to MSD involve monoclonal antibodies (e.g., IgG orIgM) to the above-described fastidious, non-hemagglutinating envelopedRNA virus. Exemplary antibodies include SDOW 12 and SDOW 17, depositedwith the American Type Culture Collection on Mar. 27, 1992 withaccession numbers HB 10996 and HB 10997, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a noninfected, unstained cell monolayer. FIG. 1B shows thecytopathic effects observed with a monolayer of cells infected with SIRSvirus VR-2332, including small granular rounded and degenerating cellsobserved three days post-innoculation with the 6th passage of the SIRSvirus.

FIG. 2A shows the direct immunofluorescence staining of a non-infectedmonolayer of MA-104 cells. FIG. 2B shows SIRS virus VR 2332 infectedMA-104 cells with intense, often granular cytoplasmic fluorescenceobserved three days post-innoculation.

FIG. 3 shows the density gradient profile of SIRS virus purified on CsCldensity gradients. Peak virus infectivity occurs at 1.18-1.19 g/ml.

FIG. 4A shows an electron micrograph of virus particles observed in CsClgradient fractions of density 1.18-1.19 g/ml. These four particles arespherical and 60-65 nm in diameter. Two particles are “empty”, showingelectron-dense core (arrows), and the other two particles are complete.Bar=100 nm. FIG. 4B shows immuno-gold electron microscopy of SIRS viruswith hyperimmune rabbit sera and anti-rabbit IgG labeled with goldparticles. Note presence of core particle approximately 25-30 nm indiameter within the virion. Bar=50 nm.

FIG. 5 shows the temperature stability of SIRS virus at 4° C. (opentriangles), 37° C. (open circles), and 56° C. (closed circles).

DETAILED DESCRIPTION

Determination of the cause of Mystery Swine Disease (MSD) has beendifficult. According to the present invention, however, the isolationand growth of the infectious agent causing MSD has been achieved. Asused herein, “infectious agent” refers to a virus capable of causingswine infertility and respiratory syndrome. More specifically, theinfectious agent is a fastidious, non-hemagglutinating enveloped RNAvirus and zoopathogenic mutants thereof capable of causing swineinfertility and respiratory disease in swine. The isolation of theinfectious agent is a major breakthrough and discovery. It enables theproduction of vaccines, antibody sera for treatment of infected swine,and diagnostic methods.

The vaccine is composed of an inactivated or attenuated MSD infectiousagent, derived from an inoculum processed from infected swine lungtissue or other swine tissue exhibiting the characteristic lesions ofMSD. Functional derivatives of infectious agent, including subunit,vector, recombinant, and synthetic peptide vaccines, or the like, arealso envisioned. A multi-step procedure is utilized in developing theMSD vaccine. The MSD infectious agent is first obtained as an inoculumby separation and isolation from infected swine tissue, preferably thelung tissue. The MSD infectious agent is then treated using knownvaccinological techniques to form a vaccine against MSD.

The MSD infectious agent is preferably isolated as an inoculate fromlung tissue of pigs which exhibit rapid breathing due to the MSD (othertissue such as fetal tissue may also be used to recover virus). Suchpigs are destroyed and their lung tissue removed. The lung tissue isthen microscopically examined for thickened alveolar septae caused bythe presence of macrophages, degenerating cells, and debris in alveolarspaces. These characteristics indicate the presence of the MSDinfectious agent. Other swine tissue exhibiting lesions of this sort mayalso be used to isolate the MSD infectious agent.

The lung or other swine tissue is then homogenized with apharmaceutically acceptable aqueous solution (such as physiologicalsaline, Ringers solution, Hank's Balanced Salt Solution, MinimumEssential Medium, and the like) such that the tissue comprises 10percent weight/volume amount of the homogenate. The homogenate is thenpassed through filters with pore diameters in the 0.05 to 10 micronrange, preferably through a series of 0.45, 0.2 and 0.1 micron filters,to produce a filtered homogenate containing the MSD infectious agent. Asa result, the filtered homogenate contains biological particles having asize no greater than about 1.0 micron, preferably no greater than about0.2 to 0.1 micron. The filtered homogenate can then be mixed withFreund's incomplete adjuvant so that the production of antibodies can bestimulated upon injection into a mammal. This mixture can be used as aninoculum for development of MSD in swine or further study of the MSDinfectious agent.

After obtaining a filtered homogenate containing the infectious agent,the infectious agent can be inactivated or killed by treatment of thefiltered homogenate with a standard chemical inactivating agent such asan aldehyde reagent including formalin, acetaldehyde and the like;reactive acidic alcohols including cresol, phenol and the like; acidssuch as benzoic acid, benzene sulfonic acid and the like; lactones suchas beta propiolactone and caprolactone; and activated lactams,carbodiimides and carbonyl diheteroaromatic compounds such as carbonyldiimidazole. Irradiation such as with ultraviolet and gamma irradiationcan also be used to inactivate or kill the infectious agent.Alternatively, the infectious agent can be attenuated by its repeatedgrowth in cell culture from non-swine mammal or avian origin so that theability of the infectious agent to virulently reproduce is lost. Thedetails of the cell culture attenuation technique are given below.

The killed or attenuated infectious agent is then diluted to anappropriate titer by addition of a diluent adjuvant solution forstimulation of immune response. The titration is accomplished bymeasurement against MSD antibody in an immunologic test such as anELISA, RIA, IFA or enzyme substrate detection test as described below.

To produce a purified form of the infectious agent, the filteredhomogenate described above can be inoculated into a series of in vitrocell preparations. Cell preparations with mammalian organ cells such askidney, liver, heart and brain, lung, spleen, testicle, turbinate, whiteand red blood cells and lymph node, as well as insect and avian embryopreparations can be used. Culture media suitable for these cellpreparations include those supporting mammalian cell growth such asfetal calf serum and agar, blood infusion agar, brain-heart infusionglucose broth and agar and the like. Preferably the mammalian cells aremonkey kidney cells, most preferably African green monkey kidneyembryonic cells—monkey kidney cell line (MA-104).

After inoculating the cell preparation with the filtered homogenate andgrowing the culture, individual clumps of cultured cells are harvestedand reintroduced into sterile culture medium with cells. The culturefluid from the final culture of the series provides the purified form ofthe virulent infectious agent. Also, after a series of repeated harvestshave been made, the culture can be grown, the culture fluid collectedand the fluid used as an inoculum for a culture of a different cellularspecies. In this fashion, the infective agent can be attenuated suchthat the culture fluid from the differing species culture provides thepurified form of the attenuated infectious agent.

Polyclonal antibody sera can be produced through use of the infectiousagent as an antigenic substance to raise an immune response in mammals.The culture fluid or inoculum prepared as described above can beadministered with a stimulating adjuvant to a non-swine mammal such as ahorse, goat, mouse or rabbit. After repeated challenge, portions ofblood serum can be removed and antigenically purified using immobilizedantibodies to those disease specific antibodies typically found in theserum of the bled animal. Further treatment of the semi-purified serumby chromatography on, for example, a saccharide gel column withphysiological saline and collection of proteinaceous components ofmolecular weight at least 10,000 provides a purified polyclonal sera foruse in treatment.

Monoclonal antibody sera can be produced by the hybridoma technique.After immunization of a mouse, pig, rat, rabbit or other appropriatespecies with MSD containing cell culture lysate or gradient-purified MSDas described above, the spleen of the animal can be removed andconverted into a whole cell preparation. Following the method of Kohlerand Milstein (Kohler et al., Nature, 256, 495-97 (1975)), the immunecells from the spleen cell preparation can be fused with myeloma cellsto produce hybridomas. Culturation of the hybridomas and testing theculture fluid against the fluid or inoculum carrying the infectiousagent allows isolation of the hybridoma culture producing monoclonalantibodies to the MSD infectious agent. Introduction of the hybridomainto the peritoneum of the host species will produce a peritoneal growthof the hybridoma. Collection of the as cites fluid yields body fluidcontaining the monoclonal antibody to the infectious agent. Also, cellculture supernatant from the hybridoma cell culture can be used.Preferably the monoclonal antibody is produced by a murine derivedhybrid cell line wherein the antibody is an IgG or IgM typeimmunoglobulin. Example monoclonal antibodies to the infectious agentfor SIRS are monoclonal antibody SDOW 12 and SDOW 17. In addition touses discussed elsewhere in this application, monoclonal antibodiesaccording to the present invention can be employed in various diagnosticand therapeutic compositions and methods, including passive immunizationand anti-idiotype vaccine preparation.

The vaccine of the present invention is capable of preventing and curingMSD infections found in the swine population. For effective prophylacticand anti-infectious use in vivo, the MSD vaccine contains killed orattenuated MSD infectious agent and may be administered alone or incombination with a pharmaceutical carrier that is compatible with swine.The vaccine may be delivered orally, parenterally, intranasally orintravenously. Factors bearing on the vaccine dosage include, forexample, the age, weight, and level of maternal antibody of the infectedpig. The range of a given dose is 10³ to 10⁷ Tissue Culture InfectiveDose 50 per ml, preferably given in 1 ml to 5 ml doses. The vaccinedoses should be applied over about 14 to 28 days to ensure that the pighas developed an immunity to the MSD infection.

The MSD vaccine can be administered in a variety of different dosageforms. An aqueous medium containing the killed or attenuated MSDinfectious agent may be desiccated and combined with pharmaceuticallyacceptable inert excipients and buffering agents such as lactose,starch, calcium carbonate, sodium citrate formed into tablets, capsulesand the like. These combinations may also be formed into a powder orsuspended in an aqueous solution such that these powders and/orsolutions can be added to animal feed or to the animals' drinking water.These MSD vaccine powders or solutions can be suitably sweetened orflavored by various known agents to promote the uptake of the vaccineorally by the pig.

For purposes of parenteral administration, the killed or attenuated MSDinfectious agent can be combined with pharmaceutically acceptablecarrier(s) well known in the art such as saline solution, water,propylene glycol, etc. In this form, the vaccine can be parenterally,intranasally, and orally applied by well-known methods known in the artof veterinary medicine. The MSD vaccine can also be administeredintravenously by syringe. In this form, the MSD vaccine is combined withpharmaceutically acceptable aqueous carrier(s) such as a salinesolution. The parenteral and intravenous formulations of MSD vaccine mayalso include emulsifying and/or suspending agents as well, together withpharmaceutically acceptable diluent to control the delivery and the doseamount of the MSD vaccine.

The method for diagnosis of MSD is carried out with the polyclonal ormonoclonal antibody sera described above. Either the antibody sera orthe biopsied tissue homogenate may be immobilized by contact with apolystyrene surface or with a surface of another polymer forimmobilizing protein. The other of the antibody sera and homogenate isthen added, incubated and the non-immobilized material removed, forexample, by washing. A labeled species-specific antibody for theantibody sera is then added and the presence and quantity of labeldetermined. The label determination indicates the presence of MSD in thetissue assayed. Typical embodiments of this method include the enzymelinked immunosorbent assay (ELISA); radioimmunoassay (RIA);immunofluorescent assay (IFA); Northern, Southern, and Western Blotimmunoassay.

The following examples further illustrate specific embodiments of theinvention. The examples, however, are not meant to limit the scope ofthe invention which has been fully characterized in the foregoingdisclosure.

EXAMPLE 1

The MSD infectious agent may be characterized by determiningphysiochemical properties (size, sensitivity to lipid solvents, andsensitivity to protease) by treatment of the inoculum followed by theinoculation of gnotobiotic pigs to determine if the MSD infectious agentremains pathogenic.

A. Materials

Gnotobiotic pigs. Derivation and maintenance procedures for gnotobioticpigs have been described in Benfield et al., Am. J. Vet. Res., 49,330-36 (1988) and Collins et al., Am. J. Vet. Res., 50, 824-35 (1989).Sows can be obtained from a herd free of reproduction problems includingMSD. Litters with stillborn and/or mummified fetuses should not be used.

MSD inoculum (MN90-SD76-GP2, referred to herein as MNSD90-x76-L orMNSD90x76-P). Trachea, lung, turbinates, tonsil, liver, brain, andspleen can be collected from nursing pigs in a Minnesota swine herdspontaneously infected with MSD (Collins et al., Minnesota SwineConference for Veterinarians, Abstract, 254-55 (1990)). A homogenate ofthese tissues (designated MN 89-35477) has been prepared in Hank'sBalanced Salt Solution without antibiotics and 0.5 ml can beintranasally inoculated into three-day-old gnotobiotic piglets using aglass Nebulizer (Ted Pella Co., Redding, Calif.). Inoculated piglets candevelop clinical signs and microscopic lesions similar to those observedin the spontaneously infected pigs. Lungs, liver, kidney, spleen, heartand brain from these gnotobiotic pigs can be collected eight days afterthe original inoculation and pooled to prepare another homogenate. Thissecond homogenate can then be inoculated one additional time ingnotobiotic pigs. Again, the same tissues may be collected andhomogenized, except that lung tissue can be prepared as a separatehomogenate because MSD can be ideally reproduced from the lunghomogenate. This lung homogenate represents the second serial passage ofthe original inoculum (MN 89-35477) in gnotobiotic pigs (Collins et al.,71st Meeting of the Conference of Research Workers in Animal Disease,Abstract No. 2 (1990)). Two filtrates can then be prepared using 0.20 μmfilter (Gelman Sciences, Ann Arbor, Mich.) and 0.10 μm filter (Millipore15 Corp., Bedford, Mass.). These-filtrates can be aliquoted and storedat −70° C. All filtrates are free of bacteria and no viruses should beobserved on direct electron microscopy using negative stainedpreparations.

Control inoculum. Homogenates of lung tissues prepared from twomock-infected gnotobiotic pigs can be used as inoculum in control pigs.This control inoculum can be prepared as 0.20 and 0.10 μm filtrates asdescribed for the MSD inoculum.

Necropsy procedures and histopathology. Pigs can be euthanized sevendays after the original inoculation as previously described in Collinset al., 71st Meeting of the Conference of Research Workers in AnimalDisease, Abstract No. 2 (1990). Tissues can be collected, fixed inneutral buffered formalin, and processed for light microscopicexamination as described in Collins et al., Am. J. Vet. Res., 50, 827-35(1989). Specimens can be collected from turbinates, tonsil, trachea,brain, thymus, lung (apical, cardiac, diaphragrnatic lobes), heart,kidney, spleen, liver, stomach, duodenum, jejunum, ileum, ascending anddescending colon, blood and mesenteric lymph nodes. These tissues can beprocessed and then examined using a light microscope to determinewhether lymphomononuclear encephalitis, interstitial pneumonia,lymphoplasmacytic rhinitis, lymphomononuclear myocarditis or portalhepatitis is present. Lesions can be consistently observed inspontaneously infected pigs from herds with MSD inoculum (Collins etal., Minnesota Swine Conference for Veterinarians, Abstract, 254-55(1990)). Fecal contents may also be collected and examined for virusparticles as previously described in Ritchie et al., Arch. Gesante.Virus-forsche, 23, 292-98 (1968). Blood can be collected for immunologicassays and tissues and cultured for bacteria as described in Example 3.

B. Infectious Agent Isolation

Lung tissue and combined brain-spleen-liver-kidney tissues obtained froman infected piglet in an SIRS-infected herd were homogenized separately.Ten percent homogenates of tissue were used. The individual homogenateswere mixed with Minimum Essential Medium (MEM) containing gentamicin atabout 100 μg per ml. Both samples were centrifuged at about 4000×g forabout 25 minutes. The supernatant was then removed and filtered througha 0.45 micron filter. The tissue and lung homogenates were thencombined, and the combined material was used to infect various tissueculture cell lines.

1. In vitro testing. Two tests were conducted using 75 cm² plasticbottles. In test no. 1, the combined material was inoculated into twobottles of full cell sheet of each of the cell lines listed below.Additionally, to one bottle of each cell line about 2.5 mg of trypsinwas added. All other remaining conditions were the same for each bottleof cell line. Serum was not in the culture medium. The inoculum was 1ml. All bottles were held for seven days at approximately 34° C. Theresults were recorded at the-end of seven days. After freezing andthawing, a sample was taken for a second passage in the same cell line.The remaining material was frozen and stored at about −60° C.

In test no. 2, the combined material was inoculated into one bottle ofthe same cells as were used in test no. 1. However, the cell sheets wereonly 20-40 percent confluent at the time of inoculation. The mediacontained about 10 percent fetal calf serum. Again, the inoculum was 1ml, and the cultures were incubated at about 34° C. for approximatelyseven days. The results of both test no. 1 and test no. 2 are summarizedbelow:

Cell Line Used Test No. 1 Test No. 2 Bovine Turbinate (BT) − − FelineKidney (CRFK) − − Monkey (Vero) Kidney − − Monkey (Vero) Lung − − CanineKidney (MDCK) − − Porcine (PK2a) Kidney − − Mink Lung − − Ferret Lung −− Bovine Lung − − Buffalo Lung − − Bovine Kidney (MDBK) − − SwineTesticle (ST) − − Monkey Kidney (MA-104) − + Human Rectal Tumor (HRT-18)− NT Human Lung NT − + = CPE effect − = no CPE effect NT = not tested

There was no cytopathic effect observed in test no. 1 in any of the celllines evaluated. In test no. 2, however, small clumps of HA-104 cellsbegan to swell and form “weak holes” in the monolayer around the edgesof the bottle. Fluid was separated from the bottle, passed into a newbottle of MA-104 cells (again 20-40 percent cell sheet), and thensubsequently passed a third time. The cytopathic effect (CPE) becamestronger with each passage. The above-described procedures were repeatedfor the MA-104 cell line employing a full cell sheet. CPE was alsoobserved. Further testing demonstrated that the viral agent will alsogrow at 37° C. The presence of serum may be helpful for the initialisolation of the viral agent. Subsequent passages of the viral agent inthe MA-104 cell line will produce the CPE without the presence of serum.

However, more pronounced CPE is observed with the use of serum in thegrowth medium for the MA-104 cell line.

The viral agent was passaged eight times in the MA-104 cell line withgood CPE developing in three days at passage five and greater. The titerobtained is approximately 5½ logs (10^(5.5)). The viral agent will alsogrow in additional simian cell lines.

2. In vivo testing. A third passage harvest was used to inoculate twothree-day-old gnotobiotic piglets. Both piglets were exposedintranasally, one with 1 ml and the other with 2 ml. The piglets wereobserved for seven days, and then were euthanized.

Tissue samples were collected for histopathologic examinations and forrecovery of the viral agent. The histopathology report confirmed thatlung lesions in the infected piglets were identical to lung lesions frompiglets known to have SIRS. The tissue samples were processed as before,and then cultured on 20-40 percent and 100 percent monolayers of theMA-104 cell line with bovine fetal serum. The viral agent was againrecovered.

A third passage harvest was also used to inoculate sows in order toverify that the reproductive effects of the disease can be duplicatedand confirmed. Two multiparous sows were inoculated intranasally at 93days of gestation. The sows delivered litters with 50 percent stillbirthpiglets (8/13 and 6/14 stillborn/live) on days 112 and 114 of gestation,respectively. Seven of the stillborn piglets were partial mummies andthe liveborn piglets were weak and failed to nurse vigorously. The viralagent was recovered from tissues of the stillborn piglets.

The viral agent has been recovered from three herds known to have SIRS.Antibody titers to the ATCC VR-2332 agent have been identified in thesesame herds.

Although there are some differences in clinical signs, i.e., cutaneouscyanosis of the ears, tail and udder in European swine, the prevailingopinion is that the North American and European diseases are caused bythe same virus, a fastidious, non-hemagglutinating enveloped RNA virusas exemplified by the deposit ATCC VR-2332.

EXAMPLE 1A Further Infectious Agent Characterization

A. Materials and Methods

1. Cells. Crandell feline kidney (CRFK), monkey kidney (MA-104) cellswere grown at 37° C. in appropriate cell culture flasks. The CRFK andMA-104 cells were propagated in Eagle's minimum essential media (MEM)(available from Gibco Laboratories, Grand Island, N.Y.) supplementedwith 10 percent gamma-irradiated fetal bovine serum (FBS) (availablefrom JRH Biosciences, Lenexa, Kans.), 1 percent penicillin-streptomycinand 2.5 μg/ml of amphotericin B. MA-104 cells were propagated in thesame media supplemented with 10 percent FBS and 50 μg/ml of gentamicin.The FES and cells were confirmed free of bovine virus diarrhea virus(BVDV) using previously described methods of Mayer et al., Vet.Microbiol., 16, 303-314 (1988); Smithies et al., Proc. Annu. Meet. U.S.Animal Health Assoc., 73, 539-550 (1969); and Vickers et al., J. Vet.Diagn. Invest., 2 300-302 (1990).

2. The source of the VR-2332 isolate (SIRS virus). The source andisolation of the SIRS virus for this Example is set forth below. Virusused in this study was on the 5th to 7th passage in MA-104 cells withtiters of 10⁵ to 10⁶ TCID₅₀/ml.

Gnotobiotic pigs. Gnotobiotic piglets obtained by closed hysterotomywere maintained in stainless steel tubs covered by flexible filmisolators as previously described by Miniatas O. P. et al., Can. J.Comp. Med., 42 428-437 (1978). The isolators were maintained at anambient temperature of 30° C. and pigs were fed recommended amounts ofcommercial milk substitute three times a day. Fecal swabs were collectedprior to experimental inoculation and at necropsy, and were inoculatedonto sheep blood agar, tergitol-seven agar and brilliant green agar inaerobic and anaerobic atmospheres. Feces collected at necropsy were alsoexamined for viruses by negative contrast electron microscopy asdescribed by Richie et al., Arch. Gesante. Virus-forsche, supra.

Source of Inoculum. A 160-sow farrow-to-finish herd in West CentralMinnesota experienced an outbreak of MSD with typical MSD symptoms. Alive sow, live neonatal piglets and stillborn fetuses were submitted tothe Minnesota Veterinary Diagnostic Laboratory for examination includinggross necropsy, histopathology and routine microbial investigation. Aninoculuin was prepared for experimental use with several tissues fromclinically ill neonatal pigs. More specifically, two live and two dead7- to 10-day-old piglets obtained during the epizootic from the affectedherd were necropsied and specimens were collected for diagnosticexaminations. The live piglets were euthanized by intravenous injectionof euthanasia solution before necropsy. A 10 percent homogenate(MN89-35477) of brain, lung and tonsil pooled from each pig was preparedusing Hank's Balanced Salt Solution (HBSS) containing 100 IU penicillin,100 μg/ml streptomycin, and 5 μg/ml amphotericin B.

Experimental Transmission. A series of 14 gnotobiotic piglets waschallenged at three days of age with pooled tissue homogenates. Eachpiglet was challenged intranasally by use of a rubber bulb attached to aglass Nebulizer placed in front of the nares of the pig. Initially, twognotobiotic piglets were inoculated with 0.5 ml each of the unfilteredinoculun (MN89-35477), monitored for clinical signs of disease, and wereeuthanized by electrocution seven days post-exposure (PE). A 10 percenthomogenate (designated MNSD-1) of lung tissues pooled from theaforementioned gnotobiotic piglets was blind passaged by exposing eachof three gnotobiotic piglets to 0.5 ml of homogenate, one pigletreceiving 0.5 ml of unfiltered homogenate, the second receiving 0.45 μmfiltrate, and the last one receiving a 0.22 μm filtrate. The pigletswere euthanized by eight days PE and tissues were collected forhistologic examination, for further passaging in gnotobiotic piglets,and for virus isolation.

A 25 percent suspension of lung (MNSD90x76-L) and a composite of brain,liver and kidney (MNSD90x76-P) of the piglet inoculated with 0.45 μmfiltrate of MNSD-1 was prepared using phosphate buffered salinecontaining 0.5 mg/ml each of kanamycin, streptomycin, and vancomycin.Six gnotobiotic piglets were inoculated with lung homogenateMNSD90x76-L; four piglets received a 0.45 μm filtrate and two were givena 0.1 μm filtrate. Three uninfected, control gnotobiotic piglets wereinoculated, one piglet with a 0.45 μm filtrate of uninfected gnotobioticpiglet tissue homogenate in HBSS and two piglets with HBSS alone.

Virus Isolation. Tissue homogenates (MNSD90x76-L 30 and MNSD90x76-p)were centrifuged at 1500×g at 4° C. for 20 minutes. The supernatant wasdiluted 1:1 with minimum essential medium (MEM) containing 10 μg/mlgentamicin, mixed thoroughly using a Vortex mixer and recentrifuged at4500×g at 4° C. for 30 minutes. The supernatant was collected andfiltered through a 0.45 μm filter. The filtrates from lung and tissuepool homogenates were combined and inoculated onto continuous cell lineMA-104. Virus isolation was done in 75 cm² flasks with 20-40 percentconfluent monolayers of MA-104 cells containing 50 ml of MEM (pH 7.5)with 10 percent fetal bovine serum (FES). Cell cultures were maintainedat 34° C. for seven days. If no cytopathic effect (CPE) was observedwithin seven days, cultures were frozen, thawed and inoculated on MA-104cells and incubated as above.

Virus Titration. Virus titration was done in 96-well, flat-bottommicrotiter plates. Serial 10-fold dilutions of virus were prepared inMEM with 2 percent FBS. After three days, the cell growth medium wasdrained from the microtiter plates, 200 μl of the virus dilution wasplaced into each of five wells, and the plates were incubated at 37° C.in an atmosphere of 5 percent CO₂. After three days, media in wells withno CPE were replaced with MEM supplemented with 2 percent FBS (pH 7.5)and a final reading was made on the fifth day of incubation. Titers werecalculated by the method of Reed et al. in Am. J. Hyg., 27, 493-497(1938).

3. Other viruses. Attenuated poliovirus, available from Dr. RogerKoment, Department of Microbiology, University of South Dakota School ofMedicine, Vermillion, S. Dak., was propagated on MA-104 cells to a titerof 10⁸ TCID₅₀/ml and the Shope strain of pseudorabies virus, availablefrom National Veterinary Services Laboratory, Ames, Iowa, was grown onCRFK cells to a titer of 10⁵⁻⁷ TCID₅₀/ml. These viruses were used as RNAand DNA virus controls in studies to determine the nucleic acid type ofthe VR2322 isolate of SIRS virus.

4. Preparation of antisera to VR-2332 isolate. Passage five of theVR2322 isolate of SIRS virus (titer 10⁶ TCID₅₀/ml) was inactivated with0.25 percent formalin, mixed 1:1 with Freund's incomplete adjuvant, anda rabbit was injected subcutaneously with 2 ml of this suspension attwo-week intervals for six weeks. Antisera prepared two weeks after thelast injection had a 1:512 neutralizing titer.

5. Virus neutralization test (VNT). MA-104 cells were seeded inflat-bottom, 96-well microtiter plates for the VNT. Serial two-folddilutions of each serum (100 μl) were prepared in MEM diluent and mixedwith an equal volume (100 μl) of isolate VR-2332 containing 100-300TCID₅₀/100 μl. Each mixture was incubated at 37° C. for one hour, and200 μl of each serum-virus mixture was added to triplicate wells.Microtiter plates were incubated for an additional three days at 37° C.,examined by light microscopy for cytopathic effects (CPE), and theendpoint titer expressed as the reciprocal of the highest serum dilutionwhich neutralized CPE.

The following polyclonal antisera, unless indicated otherwise, wereprepared as described for the VR-2332 isolate (except viruses were notinactivated) and used in the VNT: the Miller strain of transmissiblegastroenteritis; porcine rotavirus serotype 4 (Gottfried); porcinerotavirus serotype 5 (OSU); porcine reovirus serotype 1; porcineenterovirus serotypes 1-8, available from National Veterinary ServicesLaboratory, Ames, Iowa; monoclonal antibody to porcine parvovirus,available from American Type Culture Collection, Rockville, Md.;encephalomyelocarditis virus, available from Dr. W. Christianson,Department of Clinical and Population Sciences, University of Minnesota,St. Paul, Minn.; pseudorabies virus, available from National VeterinaryServices Laboratory, Ames, Iowa; Eastern, Western and Venezuelan equineencephalitis viruses; monoclonal antibody D89 to BVDV described by Mayeret al., Vet. Microbiol., .16, 303-314 (1988); equine arteritis virus,available from National Veterinary Services Laboratory, Ames, Iowa;rubella; and lactic dehydrogenase virus, available from Dr. W. Cafruny,Department of Microbiology, University of South Dakota School ofMedicine, Vermillion, S. Dak.

6. Direct electron microscopy (DEM). Cesium chloride gradient fractionswere examined by DEM for virus particles as previously described byBenfield et al., J. Clin. Microbiol., 16, 186-190 (1982); Horzinek,“Non-15 arthropod-borne Togaviruses” (Acad. Press, London, 1981); andRichie et al., Arch. Gesante. Virus-forsche, supra, and examined on anelectron microscope (Hitachi HU12A, Hitachi, Tokyo, Japan) at apotential of 75 kV.

7. Immune electron microscopy (IEM). Immuno-gold labelling was doneusing goat anti-rabbit IgG gold colloidal particles (5 nm). Briefly,virus was concentrated at 40,000×g for 30 minutes at 4° C., the pelletwas resuspended in 50 μl of distilled water, and 25 μl was placed on apiece of parafilm. A collodion carbon-coated grid was floated on thedrop of virus for 15 minutes, blotted dry with filter paper, and placedon a 25 μl drop of rabbit anti-SIRS sera for two minutes. Grids werewashed in two changes of 0.1 percent bovine serum albumin (BSA)-Trisbuffer for five minutes each and floated for 15 minutes on a drop ofgold-labeled antirabbit-IgG. After six washes in BSA-Tris buffer for twominutes each, and two washes in distilled water, the grids werenegatively stained and examined as described for DEM.

8. Hemagglutination test (HAT). The ability of VR-2332 isolate tohemagglutinate sheep, goat, swine, cattle, mouse, rat, rabbit, guineapig, human type “O”, duck, and chicken erythrocytes was determined usingstandard methods. Two-fold dilutions of the 5th to 7th passage of theVR-2332 isolate of SIRS virus were prepared in phosphate-buffered saline(pH 7.2-7.4) in U-bottom microtiter plates. Equal volumes of a 1 percentsuspension of washed erythrocytes from each of the above species wereadded to each virus dilution, incubated at 4°, 22° or 37° C.; and readafter one to two hours when erythrocyte controls (containing no virus)had settled into a button on the bottom of the well.

9. Immunofluorescence (ImF). Indirect or direct ImF staining of isolateVR-2332 infected and uninfected MA-104 cells was done using thepolyclonal or monoclonal antibodies tested by VNT. Swine influenza,available from National Veterinary Services Laboratory, Ames, Iowa (typeA, H1N1) and hog cholera virus polyclonal antisera, available fromNational Veterinary Services Laboratory, Ames, Iowa, were also used.Scrapings of the cell monolayer were removed at 72 hours PI with asterile inoculating loop ImF staining as previously described byBenfield et al., J. Clin. Microbiol., supra. Positive control slideswere stained either with convalescent antisera from a sow (VNT titer1:256) or a monoclonal antibody (SDOW 12 or SDOW 17, described herein)to the VR-2332 isolate of SIRS virus.

10. Filtration studies to estimate the size of isolate VR-2332.Clarified-infected cell culture supernatants were filtered through 0.45μm (Schleicher and Schnell, Keene, N.H.), 0.20 μm (Schleicher andSchnell, Keene, N.H.), 0.10 μm (Millipore Products Div., Bedford, Mass.)and 0.05 μm (Millipore Products Div., Bedford, Mass.) filters.

The infectivity titer before and after filtration was determined using amicrotiter assay and a previously described method of Cottral, Manual ofStandardized Methods for Veterinary Microbiology, pp. 81-82 (CornellUniv. Press, Ithaca, N.Y., 1978). A 100-fold reduction in infectivitytiter was considered significant.

11. Gradient purification of isolate VR-2332. The VR-2332 isolate ofSIRS virus from clarified culture supernatant was concentrated bycentrifugation (Beckman SW 41 Ti rotor) at 200,000×g at 4° C. for 16hours through a discontinuous sucrose gradient consisting of 2 ml of 20,30, 40, 50, and 65 percent sucrose (wt/vol) in TNC buffer (10 mM Tris,100 mM NaCl and 2 mM CaCl, pH 7.8). Culture supernatants were alsoextracted with 1,1,2-trichlorotrifluoroethane (Sigma Chemical Co., St.Louis, Mo.), concentrated by ultracentrifugation at 100,000×g at 4° C.for one hour, and purified on cesium chloride density gradients (1.20g/ml) as described for the sucrose gradients. Fractions from eithergradient were harvested from the top, the refractive index determinedusing a refractometer, and selected fractions diluted in Hank's BalancedSalt Solution for virus titration as described above.

12. Chloroform and fluorocarbon inactivation. Equal volumes of SIRSvirus (VR-2332) and chloroform were mixed periodically for 30 minutes atambient room temperature and then centrifuged at 500×g at 4° C. for 15minutes. Similarly equal volumes of 1,1,2-trichlorotrifluoroethane andSIRS virus were mixed on a vortex for five minutes and centrifuged asdescribed for the chloroform treated virus. After centrifugation, theaqueous phase of each treated sample was harvested, diluted, and amicrotiter assay used to determine the remaining virus infectivity. A100-fold reduction in titer of treated compared to untreated virus wasconsidered significant.

13. Effects of inhibitors of DNA viruses on isolate VR-2332. Thecompounds 5-bromo-2-deoxyuridine (BUDR) and mitomycin C are knowninhibitors of the replication of DNA viruses, but do not inhibit RNAviruses with the exception of the Retroviridae (Easternbrook, Virology,19, 509-520 (1962); and Reich et al., Proc. Natl. Acad. Sci., 47,1212-1217 (1961)). Pseudorabies and poliovirus were used as the knownDNA and RNA virus Controls in this experiment. Cells (CRFK or MA-104)were seeded in 96-well microtiter plates and duplicate wells wereinoculated with 100 μl of ten-fold dilutions of pseudorables virus,poliovirus or SIRS virus, respectively. After a one-hour absorptionperiod at 37° C., media containing unabsorbed virus was removed andreplaced with either MEM (untreated virus controls); MEM supplementedwith 40 or 150 μg/ml of BUDR; or MEM containing 2, 10 or 20 μg/ml ofmitomycin C. Microtiter plates were incubated at 37° C. for anadditional four days, examined by light microscopy for CPE, and thevirus titer calculated as described previously by Cottral, supra (1978).A 1000-fold reduction in virus infectivity titer compared to theuntreated control was considered significant.

14. Temperature stability of SIRS virus. A 2 ml aliquot of virus in MEMwas incubated either at 4° C. or in water baths at 370 and 56° C. for atleast five days. Aliquots from each tube at the three differenttemperatures were collected at 15, 30, 60 and 120 minutes and then at12-hour intervals from 12 to 120 hours. Virus samples were dilutedten-fold in MEM and virus titers calculated as described above.

B. Results

1. Cytopathic effect of isolate VR-2332 on MA-104 cells. The CPE startedas small, rounded clumps of cells, which appeared to be raised above theremainder of the uninfected monolayer (FIG. 1B). The number of roundedcells increased and many cells became pyknotic and detached from themonolayer within two to four days PI. By five to six days PI, CPE wasevident in 100 percent of the monolayer. Infectivity titers varied from10⁵ to 10⁷ TCID₅₀/1 ml on the 5th and 7th serial passage of virus,respectively. No CPE was observed in uninoculated 14A-104 cells (FIG.1A).

Fluorescence was not observed in uninoculated MA-104 cells (FIG. 2A).FIG. 2B demonstrates the intense, diffuse fluorescence observed in thecytoplasm of inoculated MA-104 cells stained with either convalescentsow sera, rabbit antisera or monoclonal. antibody (prepared to theVR-2332 isolate of SIRS).

2. Effects of lipid solvents on the infectivity of SIRS virus (isolateVR-2332). Pretreatment of virus with chloroform eliminated infectivity(titer reduced from 10⁵ to <10¹ TCID₅₀/1 ml), whereas fluorocarbontreatment had no significant effect.

3. Hemagglutination. Virus did not hemagglutinate erythrocytes from 11different species irrespective of the temperature of incubation.

4. Estimation of the size of the SIRS virus by filtration. Virus titerswere unchanged after filtration through 0.45, 0.20 and 0.10 μm filters.However, infectivity titers were reduced 1000-fold (10⁵ to 10² TCID₅₀1ml) after passage through a 0.05 μm (50 nm) filter.

5. Effects of DNA inhibitors on replication of the SIRS virus. Only theinfectivity of the DNA virus, pseudorabies, was reduced by either BUDRor mitomycin C. The replication of poliovirus (RNA control) and isolateVR-2332 were not affected, indicating that the genome of the SIRS viruswas probably RNA (Tables 1 and 2).

6. Virus purification. Virus bands were not detected on sucrosegradients, and peak virus titers (10⁴ TCID₅₀/1 ml) were recovered fromfractions with buoyant densities of 1.18 to 1.23 g/ml. This peak virustiter was 1000-fold less than the infectivity titer (10⁷ TCID₅₀/ml) ofthe virus suspension before purification. Cesium chloride gradientsyielded a single, faint, opalescent band at a buoyant density of 1.18 to1.19 g/ml. Peak virus infectivity of 1.3×10⁶ TCID₅₀/1 ml, which was 130times higher than peak infectivity from the sucrose gradient,corresponded to this visible band (FIG. 3).

7. DEM of purified SIRS virus particles. Pleomorphic, but predominantlyspherical virions were observed by DEM in the CsCl fractions (1.18 to1.19 g/ml). Virions were 48 to 80 nm (average of 25 particles=62 nm) indiameter and consisted of complete, electron translucent or empty(electron dense center) particles surrounded by a thin membrane orenvelope (FIG. 4A). [A 40 to 45 nm diameter isosahedral nucleocapid withshort surface projections of about 5 nm in length was observed.] Theseparticles contained cores that were 25 to 35 nm in diameter (FIG. 4B).Virions were immuno-gold labeled if the rabbit hyperimmune antisera andgold-labeled anti-rabbit IgG were used as primary and secondaryantibodies (FIG. 4B). Virions were not gold-labeled in the absence ofthe rabbit hyperimmune antisera to isolate VR-2332.

8. Antigenic relationship of SIRS virus to antisera from other viruses.Antisera to known porcine viruses and various togaviruses did notneutralize or react with SIRS virus antigen in infected MA-104 cells.Convalescent sow and hyperimmnune rabbit antisera had neutralizingantibody titers of 1:256 and 1:512, respectively.

9. Effects of temperature on the infectivity of the SIRS virus. Virusinfectivity for MA-104 cells was reduced 50 percent after incubation for12 hours at 37° C. and completely inactivated after 48 hours ofincubation at 37° C. and 45 minutes incubation at 56° C. (FIG. 5).Infectivity was unchanged after one month incubation at 4° C. or fourmonths at −70° C. (data not shown). Lung tissue collected fromgnotobiotic pigs infected with SIRS virus and homogenized in Hank'sBalanced Salt Solution retains infectivity for pigs for at least 18months.

Results indicate that isolate VR-2332 is a fastidious,non-hemagglutinating enveloped RNA virus, which 25 can be tentativelyclassified as a non-arthropod borne togavirus belonging to an unknowngenera.

The presence of an RNA genome of the SIRS virus was confirmed by theability of this virus to continue to replicate in the presence of5-bromo-2-deoxyuridine and mitomycin C, which are known to inhibit thereplication of DNA and one family of RNA viruses (Retroviridae)(Easterbrook, supra (1962); and Reich et al., supra (1961)), but notother RNA viruses. Our provisional classification of the SIRS virus asan RNA virus agrees with the observation that this virus replicates inthe cytoplasm of the cell as indicated by the presence of virus antigensdetected by ImF.

The VR-2332 isolate of SIRS virus is heat labile, but relatively stablefor long periods of time at 4° and −70° C. The thermolability of thisvirus at 37° C. has practical applications for propagation of the virus,suggesting that growth at temperatures lower than 37° C. will producehigher virus yields. Refrigeration is sufficient for preservation ofdiagnostic specimens for virus isolation for short periods of time,otherwise the sample can be frozen for several months or longer.

EXAMPLE 2

The purest form of an inoculum with the MSD infectious agent asdetermined from experiments in Example 1 may be used to transmit the MSDagent to pregnant sows to reproduce the reproductive form of the diseasesyndrome.

Discussion. Recently, transient anorexia and premature farrowing (bothprominent clinical signs of MSD in the field) was induced in 2/2 sowsinoculated with the same MSD inoculum, which produces respiratorylesions in gnotobiotic pigs. In addition, 15/29 (52 percent) of the pigswere stillborns and the remaining 14 pigs were weak and did not nursewell. No gross or microscopic lesions were observed in the stillbornpigs or the placenta and isolation procedures to detect microbial agentsare now in progress. Therefore, the experiment described below testswhether the reproductive form of MSD can be transmitted to sows byintranasal inoculation and whether interference with fetal viabilityresults from replication in maternal tissues but not fetuses.

Experiments in Example 1 provide information on how the inoculum can betreated (filter size, organic solvent extraction and/or proteasedigestion) to provide the purest form of the MSD agent for an inoculum(i.e., viral agent). Because the MSD agent has both a respiratory formin young pigs and a reproductive form in adult swine, it is necessary toreproduce the latter form of the syndrome to further verify that theinfectious agent is the putative cause of MSD.

A 93-day gestational sow is used as the experimental animal because itis possible to experimentally induce abortion in these animalsinoculated with the MSD infectious agent. Sows can be purchased from acommercial herd free of ongoing reproductive problems and MSD. Completeepidemiologic records on this herd can be computerized and informationon gestation times, litter sizes, and average number of stillbirths canbe made available for comparative studies. Groups of three sows each canbe intranasally inoculated at 93 days of gestation with either the 0.20μm filtrate (positive controls), a pathogenic but modified inoculum asdictated by results from Example 1, a 0.20 μm filtrate of the controlinoculum (negative control), and a control inoculuin modified asindicated by results of experiments in Example 1. Each group of sows canbe housed in separate isolation rooms and examined daily until gestationis complete. Temperatures and clinical signs (anorexia, respiratoryproblems such as coughing, sneezing, panting, and increased respiration)can be noted daily. Sampling of sows can be restricted to a pre- andpost-farrowing blood sample for serology. The actual date of farrowingcan be noted and the number of stillborns, mummified fetuses, live“weak” pigs and live “normal” pigs determined. Fetuses can be examinedfor gross and microscopic lesions as described in Example 1 and fetaltissues processed for microbiologic assays as described in Example 3.The fetal sera can also be assayed for the presence of gammaglobulinsand antibodies to PPV and EMCV (Joo et al., In Proceedings of theMystery Swine Disease Committee Meeting, 62-66 (1990); and Kim et al.,J. Vet. Diagn. Invest., 1, 101-4 (1990)). Pigs born live can be observedfor one week and morbidity and mortality recorded, after which thesepigs can be euthanized and the tissues collected for light microscopicand microbiologic examination as described for the fetuses.

The 0.2 μm and the modified filtrates of the MSD inoculum are pathogenicfor sows and induce anorexia, possibly a mild fever, and prematurefarrowing with a large number of stillborn and weak pigs in each litter.This illustrates evidence that the inoculum contains the MSD agent. Sowsinoculated with the control inoculum farrow near term and have littersizes within the normal range for the herd of origin as determined fromthe available epidemiologic database on this herd. No lesions in thestillborn pigs are found and a high rate of mortality among thesurviving weak pigs within one week after birth is observed.

EXAMPLE 3

Tissue samples collected from gnotobiotic piglets inoculated with theMSD inoculum and euthanized at various times post-inoculation can beused to isolate and identify the MSD infectious agent to determine thesequential development of lesions, and to ascertain whether the MSDagent is immunosuppressive.

Immunofluorescence assays on frozen tissues and inoculated cellcultures. Immunofluorescence assays on frozen tissues and cell scrapingscan be done as previously described in Benfield et al., J. Clin.Microbiol., 16, 186-190 (1982) and Benfield et al., Am. J. Vet. Res.,49, 330-36 (1988). The frozen tissues and cell scrapings can be screenedfor PPV, EMCV (See Example 4 for definition of acronyms) and the MSDagent(s). Conjugates for PPV and EMCV are available at the South DakotaAnimal Disease Research and Diagnostic Laboratory. A hyperimmune sera ingnotobiotic pigs can be prepared from the purest form of the MSDinoculum. Two pigs can be inoculated intranasally, and then given asubcutaneous booster of the MSD inoculum in Freund's incomplete adjuvantat two and four weeks after the initial inoculation. Sera can beharvested from this pig two weeks after the last booster. A control serais also prepared in two gnotobiotic pigs using the control inoculum andthe same immunization protocol described for the MSD inocultun. Thesesera can be used as primary antibody and goat or rabbit anti-porcineimmunoglobulin conjugated with fluorescein isothiocyanate as secondaryantibody to detect MSD antigens in frozen tissue sections and cellcultures.

Serologic assays. Sera collected from control and inoculated pigs can beassayed for the presence or absence of antibody to PPV and SIV(hemagglutination inhibition), Leptospira (micro-agglutination), andEMCV (viral neutralization) (see Example 4 for definition of acronyms).Previous results indicated that serology to other common microbialagents were negative (Collins, et al., 71^(st) Meeting of the Conferenceof Research Workers in Animal Disease, Abstract No. 2 (1990)).

Immunologic assays. Tissues and blood can be collected from MSDinoculated and control pigs so that their immunological status can bedetermined. Porcine leukocytes can be isolated from peripheral blood bysingle step discontinuous gradient floatation on Histopague 1077 astaught by Pescovitz et al., J. Immunol., 134, 37-44 (1985). Cells fromlymph nodes, spleen and thymus can be spilled into single cells bymoderate mincing of the tissues and collection of the resultant cellsuspensions (Hurley et al., Cancer Res., 47, 3729-35 (1987)).

Porcine leukocyte phenotypes can be determined using a panel ofmonoclonal antibodies available through the American Type CultureCollection and Joan Lunney (USDA Beltsville, Md.). These includeantibodies for the measurement of total T cells, pCD2 (MSA4; Hammerberget al., Vet. Immunol. Immunopathol., 11, 107-21 (1986), helper/class IIMHC dependent T cells, pCD4 (74-12-4; Lunney et al., Vet. Immunol.Immunopathol., 17, 135-144 (1987), cytotoxic-suppressor/class I MHCdependent T cells, pCD8 (74-2-11; Ibid), macrophages and granulocytes(74-22-15A; Ibid), thymocytes and peripheral B cells, pCD1 (76-7-4;Ibid), and pig MHC class II antigens equivalent to human DRw (MSA3;Hammerberg et al., Vet. Immunol. Immunopathol., 11, 107-21 (1986)) andDQw (TH21A and others (VRMD, Pullman, Wash.; Davis et al., HybridomaTechnology in Agriculture and Veterinary Research, Rowman and Allanheld,121-50 (1984)). Isotype-specific monoclonal antibodies to porcineimmunoglobulins are also available (Paul et al., J. Vet. Res., 50,471-79 (1989)), and can be used at twice minimum saturatingconcentration for indirect fluorescent staining of leukocytes fromperipheral blood, lymph node, and Peyer's patches (Hurley et al., Vet.Immunol. Immunopathol., 25, 177-93 (1990)). To achieve two-coloranalysis, cells can also be stained with rPE-labeled avidin after beingtagged with biotin-bound (Pierce kit #21333) antibodies. Cells can beanalyzed by flow cytometry or a two-color analytical fluorescentmicroscope (PTI FSCAN system). Co-detection in the 488 nm laser line onthe flow cytometer or using the dual analytical fluorescent microscopecan easily be attained. Intensity of cellular fluorescence andpercentage of positive cells can also be determined.

In vitro functional assays such as lectin mitogenesis with concanavalinA, pokeweed mitogen (PWM), and phytohaemagglutinin (PHA), can beperformed as described in Hammerberg et al., Am. J. Vet. Res., 50,868-74 (1989). Antigen-specific in vitro T cell responses to lysozymecan-also be modeled after their technique. B cell proliferative assayscan be performed with E. coli and S. typhimurium LPS oranti-immunoglobulin as reported in Symons et al., Int. Archs. AllergyAppl. Immun., 54, 67-77 (1977). In vitro antibody production, inducedwith PWM, can be accomplished and quantitated as described in Hammerberget al., Am. J. Vet. Res., 50, 868-74 (1989). Macrophage production ofIL-1 after 48-hour exposure to E. coli LPS can be measured in the mousethymocyte assay (Mizel, Immunological Rev., 63, 51-72 (1982)). IL-2production by PHA-stimulated lymphocytes can be measured as described byStott et al., Vet. Immunol. Immunopathol., 13, 31-38 (1986). Anisotype-specific anti-lysozyme ELISA can be done utilizing themonoclonal antibodies to porcine immnunoglobulin isotypes (Paul et al.,Am. J. Vet. Res., 50, 471-79 (1989)).

To assess in vivo antibody production, three piglets inoculated with MSDand three inoculated with control inoculuin can be injected with a 2percent suspension of sheep erythrocytes and a 10 μg/ml solution ofbovine serum albumin at separate sites at 5, 7, 10, 14, and 24 daysafter their original inoculation. Pigs are euthanized at 24 days afterthe original inoculation, tissues are collected for histopathology asdescribed in Example 1, and blood is collected to assay for antibody.The total antibody level and the specific IgG and IgM responses to eachantigen can be measured by antigen-specific radial immununodiffusion orELISA. Antigen-specific plaque assays can be performed on spleen cellsto assess the frequency of B cell clones in the infected and controlanimals (Kappler, J. Immnunol., 112, 1271-85 (1974)).

EXAMPLE 4

Objective. The goal of this experiment is to prepare hyperimmuneantisera in a gnotobiotic pig to an isolate of MSD for use as adiagnostic reagent and for further characterization of the antigenicproperties of MSD.

Background. Previous studies done in gnotobiotic pigs at South DakotaState University in collaboration with the University of Minnesotaindicated that pooled tissue homogenates from field case MN 89-35477induced lung lesions in gnotobiotic pigs. Pooled tissue homogenates fromthese pigs have subsequently been used to produce clinical disease andrespiratory lesions characteristic of MSD in three-day-old gnotobioticpigs (Collins et al., 71st Meeting of the Conference of Research Workersin Animal Disease, Abstract No. 2 (1990) and Collins et al., MinnesotaSwine Conference for Veterinarians, Abstract, 254-55 (1990)). Lung andtissue homogenates were prepared from this second passage of theoriginal inocula in gnotobiotic pig (90 X 75) to produce a secondinocula. The second inocula can be used as inocula and antigen toproduce the hyperimmune sera in this experiment.

Procedure to Accomplish the Objective

Gnotobiotic pigs. Gnotobiotic pigs can be derived and maintained aspreviously described in Benfield et al., Am. J. Vet. Res., 49, 330-36(1988) and Collins et al., Am. J. Vet. Res., 50, 827-835 (1989).

Inoculation of hyperimmune sera. Hyperimmune sera can be prepared byinitially inoculating one gnotobiotic pig as described above. Pigs canthen be given a booster consisting of 1 ml of the second inocula and 1ml of Freund's Incomplete Adjuvant at 14 and 21 days after the originalinoculation (Harlow and Lane, 1988). The pig should then be killed andexsanguinated 14 days or later after the last inoculation. Sera shouldbe harvested and dispensed into appropriate aliquots and frozen at −20′C.

Serology. The hyperimmune sera can be tested for the presence ofantibodies to common pathogens of swine as commonly done in mostdiagnostic laboratories. This sera can be tested for antibodies toHemophilus, Brucellosis, Leptospira (6 serovars), pseudorabies (PRV),parvovirus (PPV), encephalomyocarditis virus (EMC), and Swine InfluenzaVirus (SI).

Results. The pig used for preparation of the antisera was pig #4B(experimental number 90 X 238). This pig was inoculated on Nov. 1, 1990and observed daily for clinical signs until killed on Nov. 29, 1990.Clinical signs are summarized in Table 1. Unfortunately the continuingdegenerate condition of the pig mandated that it be euthanized afteronly one booster on Nov. 13, 1990. The pig was euthanized 16 days afterthe initial booster on Nov. 29, 1990.

Serology results were negative for all, the above agents except PPV,which had a titer of 16, 384 (See Table 2). A pretitered sera on thispig was not conducted.

Samples of lung, heart, brain, kidney, colon, small intestine,turbinates, spleen, stomach and trachea were collected when the pig wasnecropsied. These samples were evaluated and it was found that the lungsfrom this pig had lesions of severe pneumonia typical of that seen withfield cases of MSD.

The results of this experiment confirm initial studies that aninfectious agent is present in the second inocula because it can induceclinical disease and lesions typical of those observed in natural casesof MSD.

TABLE 1 Date Observations of Inoculated Piglet (90 × 238)  10/29 Surgery 11/1 Pig inoculated i.n. with 0.5 ml of above inocula using Nebulizerat 5:00 p.m.  11/2 Not observed. 1011/3 This pig has 2 times as muchmilk in bowl as the control pigs. I'm not sure how well the pigs areeating or if the agent is the cause of the anorexia. Feces normal.  11/4May be a little slow but alert and strong, ½ milk left, feces soft andbrown.  11/5 9 a.m.: strong, alert, drank most of milk, feces mucoid andbrown (2). 4 p.m.: strong, alert, drank most of milk, feces mucoid andbrown (2).  11/6 9 a.m.: strong, alert, drank ½ of milk, feces brownmucoid (2). 6 p.m.: strong, alert, ½ of milk left, feces loose yellowbrown (2).  11/7 9 a.m.: strong, alert, drank most of milk, feces lightbrown mucoid (2). 3 p.m.: strong, alert, drank most of milk, feces lightbrown mucoid (2).  11/8 2 p.m.: Alert, does not drink milk as well ascontrols, slower than controls, mucoid yellow feces.  11/9 8 a.m.:Alert, still does not drink as aggressively as controls, pasty feces. 5p.m.: same observation as 8 a.m.  11/10 6 p.m.: alert, vigorous, rubssnout aggressively against feed pan, feces brown loose, not eating likecontrols.  11/11 6 p.m.: alert, vigorous, rubs snout vigorously againstfeed pan, feces pasty, not eating as well as controls, still milk inpan, by 6:30 p.m. controls had finished eating.  11/12 9 a.m.: alert,but not as aggressive as controls, after 5 minutes the controls havecleaned pans but this pig still has at least ⅔ of milk in bowl. Somesnout rubbing.  11/13 Inoculated with 90 × 75 lg & pool using nebulizer(.5 m) and sp. IFA (1 ml) at 4 p.m. Alert but not as aggressive ascontrols, still has ½ pan of milk but controls have consumed all theirmilk. No snout rubbing, feces pasty.  11/15 7 p.m.: alert but not asaggressive as controls, still east slow, rough hair coat, but gainingweight like controls.  11/16 Not much change, alert but steady decliningin activity  to 1 p.m. (11/24): respiration seems to be more rapid, haircoat is  11/24 rough, not gaining weight like controls.  11/29Euthanized - blood collected for H.I. sera. Usual times collected forhistopathology including tonsil. Blood collected for lymphocytemitogenic assays. No gross lesions noted. Set up turbinate explantcultures.

TABLE 2 Antibody Tests of Inoculated Piglet 1. Swine influenza -negative 2. Swine Encephalomyocarditis - negative 3. Swine APP -negative 4. Swine PRV - negative 5. Swine PPV - negative 6. SwineBrucella - negative 7. Swine Leptospirosa - negative 8. Swine EPI -negative

EXAMPLE 4A Monoclonal Antibody Preparation

Mouse immunization. Eight weeks before the date of hybridoma fusion,inoculate BALB/c AnN mice IP with a 1:1 suspension of antigen inFreund's complete adjuvant (CFA). The amount of antigen used isdependent on the immunogenicity and toxicity of the antigen. Use amaximum of 0.3 ml CFA per mouse. Five weeks after the initialimmunization, boost mice with antigen in CFA. One week prior to thefusion, immunize mice IP with antigen in saline. Two days before thefusion, inoculate mice IV with antigen in saline.

Myeloma cells. Maintain mouse myeloma cells (P3/NS-1/1-Ag4-1 (ATCC TIB18)) on Dulbecco's Modified Eagle Media (DMEM) with 10 percent fetalbovine serum.

Approximately 10⁷ to 10⁸ cells are required for fusion with B cellsobtained from one typical mouse spleen.

Immediately prior to hybridoma fusion, harvest myeloma cells from a logphase culture into a 50 ml centrifuge tube and pellet cells bycentrifugation at 200×g for five minutes. Remove the supernatant andwash cells twice with serum free DMEM followed by centrifugation asabove. Resuspend the pellet (containing 10⁷ to 10⁸ cells) in 1 ml serumfree DNEM.

Isolation of Spleen Lymphocytes

Euthanize the mouse by cervical dislocation and immerse in 70 percentethanol for several minutes. Remove the spleen by aseptic procedure andtransfer it to a sterile petri dish containing 5 ml cold serum freeDMEM. Use a scalpel blade to slit the spleen along the long axis andgently scrape along the length of the spleen to release the splenocytesinto the media. Use a pipette to transfer the free cells and media to a15 ml centrifuge tube, leaving the spleen casing behind. Allow thetissue debris in the tube to settle for five minutes and transfer thesingle cell suspension to a fresh tube. Centrifuge cells for fiveminutes at 200×g, discard the supernatant and wash the pellet again withcold serum free DMEM. Resuspend cells in 1 ml serum free DMEM and storecells on ice until fusion.

Fusion. Add the spleen cells to the centrifuge tube containing themyeloma cells and centrifuge for five minutes at 500×g. Remove all thesupernatant and loosen the cell pellet by tapping on the side of thetube. Keeping all reagents and cells at 37° C., add 1 ml 50 percentpolyethylene glycol solution (PEG 4000, Gibco, Grand Island, N.Y.)dropwise to the tube over a one-minute period with gentle mixing. Allowthe mixture to stand at 37° C. for one minute. Add 1 ml warm serum freeDMEM dropwise over one minute with gentle mixing. Finally, add 20 mlserum-free DMEM dropwise over four minutes, then immediately centrifugecells at 200×g for five minutes. Discard the supernatant and resuspendcells in 47 ml DNEM containing 20 percent fetal bovine serum, 0.2units/ml insulin, 0.5 mM sodium pyruvate, 1 mM oxaloacetic acid, 2mM-glutamine, non-essential amino acids, and 10 percent NCTC-109lymphocyte media. Add 1 ml volume of cell suspension to two wells of two24-well, flat-bottom tissue culture plates. Include a myeloma cellcontrol well and incubate plates at 37° C. and 10 percent CO₂ (SDMEM).

Following overnight incubation, remove 0.5 ml of 10 media from each wellwithout disturbing the cell layer. Add 1 ml SDMEM containing 1×10⁴ Mhypoxanthine, 4×10⁷ M aminopterin, and 1.6×10⁻⁵ M thymidine (HAT) toeach well and continue incubation at 37° C. and 10 percent CO₂. Continueto replace 1 ml of used media with 1 ml of fresh 15 DMEM+HAT three timesweekly for two to three weeks. When significant clone growth isapparent, assay the wells for the presence of specific antibody byELISA, indirect PA, or other appropriate assay systems.

Cloning. Primary wells testing positive for specific antibody should besubcloned immediately to obtain a stable cell line and avoid overgrowthby other clones. Resuspend cells from selected primary wells and performcell counts using trypan blue stain and a hemocytometer. Make dilutionsof the cells to obtain a final concentration of about 2 cells/ml inSDMDM+HT. Use normal spleen cells obtained from non-inoculated mice as afeeder layer by adding 50 μl packed cells-per 100 ml media.

Add 250 μl of cell suspension to each well of 96-30 well plates andincubate at 37° C. and 10 percent CO₂.

Clones should be visible in two to three weeks and supematants fromwells containing single clones are assayed when significant growth isapparent. Repeat the cloning procedure with wells testing positive forspecific antibody. Slowly expand selected clones to tissue cultureflasks for further characterization and cryopreservation.

As cites Production. Prime BALB/c AnN mice with 0.5 ml pristane given IPtwo weeks before inoculation with hybridoma cells. Harvest hybridomacells and wash once with Hank's Balanced Salt Solution (HBSS). Resuspendcells in HBSS and inoculate primed mice IP with 10⁴ to 10⁶ viable cells.When ascites production is apparent (usually one to two weeks afterinoculation) drain by inserting a 16 G 1½″ needle ventrally in theinguinal region. Hold the hub of the needle over a centrifuge tube anddrain ascites into the tube. Centrifuge ascites fluid at 200×g, filterthrough a 0.2 nm filter, and store frozen.

EXAMPLE 4B SIRS Monoclonal Antibodies (SDOW 12 and SDOW 17)

The SDOW 12 (ATCC No. HB 10996) and SDOW 17 (ATCC No. HB 10997)monoclonal antibodies to the SIRS virus were prepared using the abovestandard monoclonal antibody production protocol. The antigen used formouse immunization was passage 6 SIRS virus (VR-2332) grown on MA-104cells obtained from Boehringer Ingelheim Animal Health (BLAH). For eachimmunization, mice were inoculated with 0.3 ml of gradient purifiedvirus with a titer of 10⁶ TCID₅₀/100 μl.

An indirect fluorescent antibody (IFA) assay was used to detect specificantibody in hybridoma primary wells and clone wells. Acetone fixed virusinfected and non-infected cell monolayers in 96-well tissue cultureplates were used for the IFA. Cell culture supernatants and ascitesfluids from these hybridomas produced bright, granular cytoplasmicfluorescence in SIRS infected cells.

Preliminary characterization of these monoclonal antibodies includedimmunoglobulin isotyping and radioimmunoprecipitation of SIRS viralproteins. The SDOW 12 and SDOW 17 monoclonal antibodies are both of theIgG₁ isotype. They also both bind to a 15 kD viral protein onradioimmunoprecipitation.

EXAMPLE 5

Three pilot studies are described. A gnotobiotic pig study wasundertaken to show that field material could be used to infect and causethe respiratory component of the syndrome in germ-free pigs. A secondstudy using conventional weaned pigs was undertaken to determine if therespiratory disease seen in gnotobiotic pigs could be reproduced inconventional pigs. And finally, a pregnant sow study was undertaken todetermine if the reproductive failure component of the syndrome could beexperimentally reproduced.

Material and Methods

Field case. See Source of Inoculum in Example 1A.

Gnotobiotic study. Six hysterectomy-derived gnotobiotic piglets wereinoculated intranasally at three days of age with the field inoculum (10percent homogenates, various tissues). Filtered (0.22 μm) and unfilteredinoculum were used. Two control piglets were inoculated with media only.Clinical signs were monitored daily and the pigs were euthanized eightdays post-inoculation, except one animal which was held for theproduction of hyperimmune serum. Tissues for virus isolation andhistologic examination were collected at necropsy along with sera whichwas screened for antibodies to leptospira, chiamydia, eperythrozoon,Aujeszky's disease virus, porccine parvovirus, encephalomyocarditisvirus, hemagglutinating encephalitis virus, swine influenza virus,bovine respiratory syncytial virus, canine distemper virus, bovine viraldiarrhea and hog cholera. The original inoculum and tissues fromgnotobiotic piglets were inoculated onto continuous and primary celllines for three passages. In addition, direct and immunoelectronmicroscopy was performed.

Conventional pig study. Three conventional 28-10 day-old weaned pigsfrom a farm with no history of MSD were intranasally inoculated with 10percent lung homogenates from affected gnotobiotic piglets. A lunghomogenate from a negative gnotobiotic pig was used as inoculum for acontrol. Piglets were monitored daily for clinical signs and werenecropsied eight days post-inoculation. Sera/tissueg were processed asdescribed above.

Pregnant sow study. Eight multiparous sows with known historical duedates from a farm free of MSD were used in this study. Three weeks priorto the expected farrowing date, six sows were intranasally inoculatedwith affected gnotobiotic lung homogenates and two sows with negativelung homogenates. Clinical signs were monitored daily. The sows wereallowed to farrow naturally and when possible the farrowings wereattended in order to collect presuckled sera from live born pigs. Sowsand live pigs were euthanized shortly after farrowing and tissues werecollected for histopathology and virus isolation. Sera or fetal thoracicfluids were also collected.

Results

Field study. No gross lesions were seen at necropsy. Microscopicexamination of nursing piglets revealed necrotizing interstitialpneumonia and lymphomononuclear encephalitis. The fetuses did not havelesions but the sow did have a mild encephalitis. Microbiologicexamination did not yield conclusive results.

Gnotobiotic study. The piglets became anorexic and developed rough haircoats three days post-inoculation. Controls remained normal. Microscopiclesions were found in the principals inoculated with filtered ornon-filtered material. The lesions were similar to field cases andincluded: necrotizing interstitial pneumonia (6/6 [six out of sixpiglets]), lymnphoplasmacytic rhinitis (4/6), lymphomononuclearencephalitis (2/6) and myocarditis (1/6). No etiologic agent wasidentified either by pre- and post-inoculation serology or throughinoculum/tissue examination.

Conventional weaned pig study. Clinically, the principals became dulland anorexic two days post inoculation. The animals appeared chilledeven though adequate heat was provided. One pig had an elevatedtemperature (41.5° C.) six days post-inoculation. Interstitialpneumonia, encephalitis and myocarditis were found in the principals butnot the control.

Pregnant sow study. Clinically, only two principals showed anysignificant temperature rises (1.5° C.) day three or five post-inoculation. However, anorexia was noted in 4/6 sows at day four or fivepost-inoculation. Three sows farrowed up to seven days early and threesows farrowed on time. Over 50 percent of the fetuses from infected sowswere born dead while the controls had normal litters. Both stillbornsand late-term mummies were found in infected litters. Laboratoryfindings were not conclusive—no specific agent has been identified andno lesions have been noted in fetuses to date.

Although no causative microorganism has been identified, the findingssuggest MSD can be transmitted experimentally to gnotobiotic andconventional pigs (using field tissues from one farm). Both therespiratory and reproductive forms of MSD were reproduced. The agentinvolved appears infectious, filterable at 0.22 μm and is seeminglyfastidious.

Discussion

MSD is an important emerging disease not only in the United States butalso throughout the world. In order to study the disease in a controlledsetting, gnotobiotic pigs were inoculated intranasally at three days ofage with tissue homogenates from a farm experiencing the clinical signsof MSD. Microscopic lesions similar to field cases including necrotizinginterstitial pneumonia and to a lesser extent lymphoplasmacyticrhinitis, lymphomononuclear encephalitis or myocarditis were seen inprincipals but not controls. Using lung homogenates from the gnotobioticpigs, intranasal inoculation of conventional four-week-old weaned pigsproduced similar lesions. Multiparous pregnant sows were also inoculatedwith gnotobiotic lung homogenates three weeks prior to their due dates.Clinically these sows went through a period of anorexia and farrowed upto seven days early. Over 50 percent of the fetuses were eitherstillborn or in the beginning stages of mummification. The findingsindicated the disease associated with the infectious agent can beisolated and transmitted experimentally with field tissues tognotobiotic pigs and from gnotobiotic pigs to conventional weaned pigsor pregnant sows. This study provides a model for both the respiratoryand reproductive forms of the disease which will lead to furtherinvestigations of the pathogenesis and diagnosis of MSD.

1. An immunogenic composition comprising attenuated SIRS virus ATCCVR-2332 which is non-zoopathogenic in swine.
 2. The immunogeniccomposition of claim 1, further comprising a pharmaceutically acceptablecarrier.
 3. The immunogenic composition of claim 1 comprising 10³ to 10⁷TCID₅₀/ml of the attenuated SIRS virus ATCC VR-2332.
 4. The immunogeniccomposition of claim 1 comprising 10⁵ to 10⁷ TCID₅₀/ml of the attenuatedSIRS virus ATCC VR-2332.
 5. The immunogenic composition of claim 1comprising 10⁵ to 10⁶ TCID₅₀/ml of the attenuated SIRS virus ATCCVR-2332.
 6. The immunogenic composition of claim 1, wherein theattenuated SIRS virus is produced by a process comprising passaging SIRSvirus ATCC VR-2332 through simian cells to form a modified SIRS viruswhich is non-zoopathogenic in swine.
 7. The immunogenic composition ofclaim 6, wherein the simian cells are simian kidney cells.
 8. Theimmunogenic composition of claim 7, wherein the simian kidney cells areMA-104 simian kidney cells.
 9. The immunogenic composition of claim 1,wherein the attenuated SIRS virus is purified by gradient or serial cellculture.
 10. A method of inducing an immune response against swineinfertility and respiratory syndrome virus comprising administering thecomposition of claim 1 to a mammal.
 11. The method of claim 10, whereinthe mammal is a swine.
 12. A method of inducing an immune responseagainst swine infertility and respiratory syndrome virus comprisingadministering the composition of claim 2 to a mammal.
 13. A method ofinducing an immune response against swine infertility and respiratorysyndrome virus comprising administering the composition of claim 3 to amammal.